Anti-locking control system

ABSTRACT

An anti-lock brake system is described in which, when an instability limit IS is exceeded by an instability criterion K 
     
         K=aB+b·L+cIL+dV.sub.F 
    
     (where a, b, c and d are constants and B is the relative wheel deceleration, L is the wheel slip and IL is the integral of the wheel slip and V F  is the calculated vehicle deceleration, a pressure reduction pulse is produced, the width of which depends on the reduction time in the preceding cycle. This pulse also resets the integrator and the differentiator required for obtaining the wheel deceleration to 0. Between the pressure reduction pulses, pressure is held constant.

BACKGROUND OF THE INVENTION

German Offenlegungsschrift 3,614,770 discloses an anti-lock brake systemin which the wheel deceleration and the wheel slip are integrated for apredetermined time and the results of integration summed. If the sumexceeds a limit, a pressure reduction pulse is produced and the resultsof integration are at least partially erased; preferably, apredetermined number of measured values are integrated, the measuredvalue already taken into consideration the longest being oxcluded upontaking into consideration a new measured value (time window).

SUMMARY OF THE INVENTION

The design of the anti-lock controller creates a convenient controllerwhich reacts rapidly to corresponding driving states.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the controller,

FIG. 2 shows details of the controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, wheel-speed sensors 1 and 2 are assigned to the non-drivenwheels. From the speed values of these sensors 1 and 2, a speedcharacteristic approximated to the vehicle speed V_(P) is produced in ablock 3. Blocks 4 and 5, to which both the wheel-speed signals V_(R) andthe vehicle-speed signal V_(F) are fed, each emit an output signal whichcorresponds to the slip L of the wheel. These signals are fed tocomputers 6 and 7, but also to integrators 8 and 9 which integrate theslip signal supplied and likewise feed the integration signal IL to thecomputers 6 and 7 respectively.

In differentiating elements 10 and 11, the wheel-speed signals V_(RR)and V_(KL) from sensors 1 and 2 are differentiated; the output signalsare then fed to subtractors 12 and 13, to which the calculated vehicledeceleration V_(F), from differentiatior 14 is also fed. In blocks 12and 13, the relative wheel deceleration B=V_(R) -V_(F) for both wheelsare formed and likewise fed to the computers 6 and 7 respectively. Thecomputers 6 and 7 respectively produce for the wheel assigned to themthe sum aB+bL+cIL+dVF, a, b, c and d being weighting constants. If theinstability criterion K exceeds an instability limit IS, a pressurereduction pulse is produced at the output of the computer 6 or 7, whichpulse is fed to the valves assigned to the wheels (inlet valves 15a or16a and outlet valves 15b or 16b ) for the purpose of pressure reduction(both valves 15a and 15b or 16a and 16b are activated).

The pressure reduction pulse is also used to reset the integratingelements 8 or 9 and differentiating elements 10 or 11 to zero. Betweenthe pressure reduction pulses, the pressure is held constant.

The width of the first pressure reduction pulse in a cycle isdimensioned so that is corresponds to a certain proportion of the sumABZ of the pressure reduction times of the preceding cycle. Thesubsequent pulses represent a fraction of the first pulse; dimensioningis effected by the computers 6 and 7 respectively. In order to achieve alinearisation of the pressure reduction times, the sum of the pressurereduction times ABZ is converted in the computer 6 and 7, respectively,into an equivalent activation time ABZ* in accordance with therelationship ABZ*=ABZ (1-e^(t/T)). Here, t is the activation time of theoutlet valve and T is a time constant matched to the hydraulics.

The computers 6 and 7 also determine in a known manner the instant atwhich, before reaching the maximum slip, the wheel speed reaches itsturning cusp point (=cusp point of the wheel deceleration) and nowchanges the instability relationship in such a way that this isdetermined more by the wheel deceleration and less by the slipcomponents. The relationship can, for example, be K=B+L+IL/2. As soonas, by comparison of successive measured values of the wheel speed, thecomputer detects the tendency to an increase in wheel speed, it outputsa signal via a line 18 or 19 to a block 20 or 21 which then interruptsthe activation of both outlet valves 15b and 16b (pressure reduction).However, via line 22 or 23, the inlet valve 15a or 16a remains activated(holding constant).

The computer 6 or 7 preferably dimensions the length of the first pulsein the first control cycle in proportion to the wheel deceleration andin inverse proportion to the vehicle deceleration.

Under certain conditions, a prolongation of pressure reduction isperformed until the occurrence of a certain relative wheel acceleration+B. More specifically, a prolongation is performed when, despite atendency to an increase in wheel speed, the differentiator 10 or 11still outputs a deceleration value, i.e. when the differentiator doesnot follow. This condition can also be made dependent on whether thealready accumulated pressure reduction time ABZ' exceeds a certainvalue. An implementation of this condition is shown in FIG. 2. Accordingto the circuit of FIG. 2, the prolongation of pressure reduction iscontained in block 20 or 21. The reduction signal is fed to the ANDelement 202 via the terminal 201. It passes via an OR element 203 to thevalves 15 or 16 until the signal +Δv (tendency to an increase in speed)occurs.

In the special case that, despite the tendency to an increase in speed(+Δv present), a (small) wheel deceleration (B<-X) is still detected andpressure has already been reduced for at least a predetermined time(ABZ'>y), an AND element 204 produces a signal which sets a bistableelement 205. This then outputs a pressure reduction signal to the ORelement 203. If a +B signal occurs, it resets the bistable element 205again.

Following a pressure reduction, the pressure is held constant until alimit ST is exceeded by a stability criterion K_(st) =e·B*+fL. Here, B*is the relative wheel acceleration B*=V_(R) -V_(F), L is the (negative)slip and e and f are constants.

The pressure reduction is preferably effected in a pulsed fashion,initially with a large gradient and then with a reduced gradient. Theperiod of time after which a changeover is effected to the reducedgradient is preferably made dependent on the length of the precedingpressure reduction and, in addition, on the ratio of the pressurebuild-up time to the pressure reduction time in the preceding cycle. Thepressure build-up pulses on the line 22 or 23 are dimensioned by thecontroller 6 or 7.

If a very high wheel acceleration occurs, a pressure build-up pulse isproduced which feeds in the previously present brake pressure again.

We claim:
 1. Antilock brake system for a vehicle having wheels, saidsystem comprisingvalve means including an outlet valve for regulatingbrake pressure at at least one wheel in control cycles, each cycle beingcharacterized by a sum ABZ of pressure reduction times, means for (1, 2)for determining speed V_(R) of said at least one wheel, means (10, 11)for differentiating the speed V_(R) to produce a deceleration V_(R) ofsaid at least one wheel, means (3) for determining the vehicle speedV_(F), means (14) for determining the vehicle deceleration V_(F) fromthe vehicle speed V_(F), means (4, 5) for determining the wheel slip Lof said at least one wheel from the wheel speed V_(R) of said at leastone wheel and the vehicle speed V_(F), means (8, 9) for integrating theslip L to produce an integral IL for said at least one wheel, means (12,13) for determining the relative wheel acceleration B=V_(R) -V_(F) forsaid at least one wheel, means (6, 7) for forming an instabilitycriterion K=aB+bL+cIL+dV_(F), where a, b, c, and d are constants, means(6, 7) for comparing K to an instability limit IS and, when K>IS,producing a first pressure reduction pulse of a duration dependent uponthe sum ABZ of reduction times in the preceding cycle, resetting themeans (8, 9) for integrating the slip, and resetting the means (10, 11)for differentiating the wheel speed, said pulse acting on said valvemeans to reduce brake pressure at said at least one wheel, whereuponsaid valve means are controlled so that the brake pressure remainsconstant.
 2. Anti-lock brake system according to claim 1, characterizedin that the duration of the first pressure reduction pulse in one cyclecorresponds to a substantial fraction of the sum ABZ of the pressurereduction times of the preceding cycle and in that subsequent pressurereduction pulses are produced following said first pressure reductionpulse in said cycle, said subsequent pulses having a duration which is afraction of the duration of the first pulse.
 3. Anti-lock brake systemaccording to claim 1, characterized in that the sum of the pressurereduction times ABZ of the preceding cycle is converted before producingthe first pressure reduction pulse in accordance with the equation

    ABZ*=ABZ(1-e.sup.t/T)

where t is the activation time of the outlet valve and T is a timeconstant matched to the hydraulics.
 4. Anti-lock brake system accordingto claim 1 wherein when a turning cusp point of the wheel speed isreached before a maximum slip value is reached, the instabilitycriterion K is altered to depend more on the wheel deceleration B thancomponents L, IL, and V_(F).
 5. Anti-lock brake system according toclaim 1 wherein the pressure reduction is interrupted as soon as atendency to an increase in the wheel speed is detected.
 6. Anti-lockbrake system according to claim 1 wherein the duration of the firstreduction pulse of the first control cycle is proportional to a wheeldeceleration and inversely proportional to the vehicle deceleration. 7.Anti-lock brake system according to claim 1 wherein, characterised inthat a pressure reduction is carried out until a positive threshold ofthe wheel acceleration B is reached if, despite the detection of atendency to an increase in the wheel speed, the wheel acceleration stillfalls below a predetermined value.
 8. Anti-lock brake system accordingto claim 7, characterised in that the prolongation of pressure reductionis only operative if the pressure reduction time has exceeded apredetermined duration.
 9. Anti-lock brake system as in claim 1 whereinthe pressure is held constant after being reduced and a stability limitST is predetermined, to which is compared the stability criterion K_(ST)which is formed according to the relationship

    K.sub.ST =eB*+fL

where e and f are constants, B* is the relative wheel acceleration and Lis the negative brake slip, and wherein a pressure build-up is triggeredif the stability limit ST is exceeded.
 10. Anti-lock brake systemaccording to claim 9, characterised in that the pressure build-up iseffected in a pulsed fashion.
 11. Anti-lock brake system according toclaim 10, characterised in that, at the beginning of the brake-pressurebuild-up, an increase in the brake pressure is effected with a gradientfor a period of time and, after this, with a smaller gradient, theperiod of time being dependent on the length of the directly precedingpressure reduction, and in that the period of time is, in addition,dependent on the ratio of the pressure build-up time to the pressurereduction time in the preceding control cycle.
 12. Anti-lock brakesystem according to claim 11, characterised in that the period of timein the first control cycle is dependent on the preceding pressurereduction time.
 13. Antilock brake system as in claim 1 wherein saidvehicle has driven wheels and non-driven wheels, said means fordetermining speed V_(R) determining the speed of the non-driven wheels.14. Antilock brake system as in claim 13 wherein the vehicle speed V_(F)is determined from the speeds of the non-driven wheels.